1. Field of the Invention
This invention relates generally to communications systems, and, more particularly, to a method and apparatus for roaming on a wireless network.
2. Description of the Related Art
This invention relates to wireless data communications networks, and in particular to arrangements for communications between mobile data handling units and a central computer using wireless data communications.
The assignee of the present invention supplies a wireless data communications system known as the Spectrum24® System, which follows the radio data communications protocol of Institute of Electrical and Electronics Engineering (IEEE) Standard 802.11. In the system as implemented, mobile units are in data communication with a central computer through access points. The access points may communicate with a central computer or several computers over a wired network. Each of the mobile units associates itself with one of the access points.
The access points in this system perform the duties in the standard protocol, including, association, roaming, packet formulation, parsing, packet fragmentation, packet re-assembly, encryption, and system access control. To maintain order and reduce radio communications each access point determines which of the data communications received over the wired network from the central computer is addressed to a mobile unit associated with that particular access point. This requirement adds a significant computational burden to the access point, increasing the cost thereof. In addition, in applications that must support a high volume of data communications from multiple users, such as a self service shopping system, a hospital system, a systems that includes paging or voice data links to many users, or a system supporting communication with electronic shelf labels, additional access points are required to support the data communications traffic, increasing the overall system cost. The cost of an operational access point is dependent not only on the complexity thereof and the requirement for high speed processing of data packets for purposes of selecting those destined for mobile units associated with an access point, but also on the additional cost of the installation of electrical power to the location of the access point and the cost of a power supply to convert AC electrical power to DC power for the circuits of the access point. Further cost may be involved in physically mounting the access point hardware and antenna.
Current 802.11 implementations employ a lengthy, 12-15 packet exchange between a mobile unit and an access point when the mobile unit roams to the access point. Each access point operates independently of the others from a security standpoint. Hence, if the mobile device roams and wishes to transfer between access points, the lengthy association exchange is repeated between the mobile device and the new access point. The size of the exchange significantly affects the time required for a mobile unit to associate with access points. In latency sensitive applications, such as multimedia or voice over internet protocol (VOIP), the quality of the service may be affected by the significant roaming delay. Service interruptions may occur during the roaming process.
In prior systems, as discussed above, each access point is connected on a wired network (e.g., Ethernet) to the central computer. The access points determine the identity of mobile units that have become associated with them and extract from the data packets on the network those packets addressed to each mobile unit associated with the access point. This procedure generates a significant processing burden for the access points that increases cost for the access points.
Prior systems may also have a large number of access points, each with a memory containing program instructions for carrying out the various required functions. This distribution of program instructions makes it difficult to upgrade a system or to provide changes in system configuration, because any upgrade or change may require changes to the program instructions in each of the access points. Such distribution of processing functions also makes system management functions, such as load balancing or access control, more difficult.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.